1. Field of the Invention
This invention relates generally to recycling of polystyrene based products, and reclamation of styrene monomer from products containing polystyrene. The invention more particularly concerns a method for depolymerizing polystyrene producing a yield of styrene monomer and other useful polystyrene depolymerization by-products.
2. Description of Related Art
Polystyrene is a thermoplastic resin that is useful in many applications, for packaging, thermal and electrical insulation, fast food takeout containers, egg cartons, cups, coolers, and a variety of other types of containers and packaging products. Polystyrene is available as foam, also sold under the trademark "Styrofoam," owned by the Dow Chemical Co., which is useful for thermal insulation in light construction, such as for boats or trailer homes or the like, and in the form of foam beads, spheres or other shapes which are useful in packing. Polystyrene is also injection molded in the manufacture of toys, furniture, and medical products such as petri dishes and bioassay equipment. Because of the large amount of such forms of polystyrene which is largely discarded, it would be useful to provide a method of recycling such common commercial polystyrene products.
Polystyrene is also a major ingredient of napalm, which is a solution of polystyrene in benzene and gasoline. Recycling of the large existing military weapon stores of napalm by distillation of benzene and gasoline to yield polystyrene presents a number of dangers and difficulties, which has heretofore made this direct approach to reclaiming polystyrene from napalm inefficient and largely impractical. When dissolved in solvents such as benzene and gasoline and boiled, polystyrene acts as an effective foaming agent. In addition, a solution of polystyrene in such solvents is generally highly viscous, and has a low coefficient of heat transfer. When napalm reaches approximately 40.degree. C., the lighter fractions of gasoline begin to boil off, and at approximately 80.degree. C., the benzene boils. The free escape of benzene and gasoline gases being distilled from napalm is inhibited by the high viscosity of the solution, which becomes increasingly viscous as the solvents are distilled out of the solution. The boiling napalm can create a heavy foam, also referred to herein as the "souffle effect." This foam can extrude into exits of the distillation apparatus, where the foam can cool, forming a solid polystyrene, blocking the exits and allowing pressure and temperature within the distillation apparatus to build up dangerously.
A "souffle effect" from heavy foaming can also occur at a later stage in distillation when all of the benzene and light gasoline components have evaporated from the solution, leaving heavier gasoline components to boil off through a more viscous solution at only a slightly higher temperature. High viscosity further inhibits convective mixing and uniform heating, so that destructive "hot spots" can occur at the interface of the liquid with the surface of the distillation apparatus.
The heavy foaming which can occur during distillation of napalm can be reduced to some extent by the use of expensive and complex high pressure distillation systems or by distilling slowly. However, polystyrene also undergoes a degree of depolymerization not only as a function of temperature, but also as a function of the duration of exposure to elevated temperatures, producing styrene monomer and partial depolymerization products which are unacceptable in recovered polystyrene plastic. Some formulations of gasoline contain high boiling point fractions similar to kerosene. At high temperature, these fractions can react with the polystyrene plastic to form compounds which have disagreeable odors, and are therefore unacceptable contaminants in recycled polystyrene plastic. Driving out the residual gasoline fractions by further elevation of temperatures only causes more rapid depolymerization of the polystyrene plastic and further contamination of the polystyrene plastic by partial depolymerization products.
In view of the difficulty of extracting clean polystyrene intact from napalm, it would be advantageous to depolymerize the polystyrene to yield styrene monomer, which is a useful end product, and which can be easily recovered at commercial purity. Such a process would be useful for recovering styrene monomer from polystyrene products, such as foamed polystyrene, solid polystyrene, and napalm. One conventional process provides for depolymerizing polystyrene in a solvent such as benzene, having a boiling point substantially different from that of styrene, under temperature conditions of 350.degree. C. to 600.degree. C., but in sufficiently small quantities and the shortest possible amount of time, preferably less than one minute, to avoid production of high boiling point oils, considered in that process to be undesirable contaminants, and other undesirable by-products. The styrene production rate of that process is therefore uneconomically low.
It would be desirable to provide a simple fractional distillation method requiring no pressure vessels or piping and no complex separation equipment to permit the reclamation of large quantities of styrene monomer from polystyrene based materials such as napalm or foamed styrene products. It would also be desirable to provide a process for reclaiming styrene monomer from such polystyrene materials that would prevent the occurrence of the heavy foaming "souffle effect", and that would avoid contamination of the yield by residual fractions.
The recycling and reclamation of foamed polystyrene plastic has further presented a serious environmental challenge in recent years. Although some recycling of large, clean pieces of polystyrene foam is currently performed by cutting them into small pieces suitable for use as packing materials, there is limited demand for this type of recycled foamed polystyrene, and much waste polystyrene currently goes unreclaimed. A further problem is presented by the extremely low density of polystyrene foam, such that a whole truckload of polystyrene foam hardly weighs enough to pay for transportation to a reprocessing plant.
It would also be desirable to provide a process that greatly increases the density of the polystyrene material to be recycled, at the places of collection sites, to allow economical transportation of the collected polystyrene in sufficient quantity and density to allow conversion of the material into usable, salable materials. In addition, the reclamation process should be able to accept large and small pieces of polystyrene, and to tolerate some foreign matter, water and dirt mixed in with the plastic to be reclaimed. The present invention meets these needs.